Hitherto known as a technique of generating or detecting terahertz waves is terahertz time-domain spectroscopy (THz-TDS). As known in the art, the terahertz time-domain spectroscopy is suitable for use in imaging samples because the THz-TDS utilizes terahertz waves that define ultra-short pulses, as short as about 100 femtoseconds. Therefore, the terahertz time-domain spectroscopy attracts attention in various technical fields such as industry, medical care, biotechnology, agriculture and security.
In a THz-TDS system, radiation emitted from an ultra-short pulses laser source is split into a pump beam and a probe beam. The pump beam is focused on a terahertz-wave generating element. In the terahertz-wave generating element, a current flow or electrical polarization with a sub-picosecond duration 1 generates a terahertz wave having a field amplitude proportional to the temporal differentiation. The terahertz wave is focused by an optical system on a terahertz-wave detecting element. At this point, the probe beam is applied to the terahertz-wave detecting element. Then, the terahertz-wave detecting element generates a carrier. The carrier is accelerated by the electrical field of the terahertz wave and changed to an electric signal. The time at which the probe beam reaches the terahertz-wave detecting element is delayed, thereby measuring the time waveform the terahertz wave has in the amplitude electric field. The time waveform is Fourier-transformed, thereby determining the spectrum of the terahertz wave.
An apparatus using the terahertz time-domain spectroscopy has been proposed (see, for example, WO 01/06915). In this apparatus, the optical path for the pump beam, which extends from the beam splitter to the terahertz-wave generating element, and the optical path for the probe beam, which extends from the beam splitter to the delay unit are optical fibers.
It has been proposed that some of the spatial optical components of the fiber laser, such as polarizing beamsplitter (PBS), wavelength plate, and collimator lens, should be replaced by optical fibers in order to miniaturize the fiber laser. In the terahertz field, too, there is a strong demand for smaller spectrometers. However, terahertz spectrometers tend to be large because they need optical elements for guiding the pulse beam from the fiber laser to the beam splitter.